Processing or developing photographic film and paper products and other imaging products requires the use of a variety of known types of processing solutions. During use, the processing solutions gradually lose their effectiveness and must be replaced with fresh solutions. Photographic processors and film manufacturers for many years have been concerned with how to properly dispose of the spent or waste solutions. The spent solutions may contain precious metals such as silver which often have been recovered as an economic measure. Also, government regulations on discharge to the environment of solutions containing such metals typically have required that virtually all of the metals be removed before the remaining liquid may be discharged to the sewer. Certain ingredients in such spent solutions have been reacted with precipitating and flocculating agents to form solid precipitates containing the metal or other ingredient to be removed. The precipitates have been filtered from the solution and the remaining liquid has been discharged. Some film processors have separated the precipitates by centrifuging. Others have separated the precipitates by simple settling and decanting. Various techniques of these sons are described in commonly assigned U.S. Pat. No. 3,832,453 and by Thomas W. Bober and Austin C. Cooley in "The Filter Press for Filtration of Insoluble Photographic Processing Wastes," Photographic Science and Engineering, Vol. 16, No. 2, March-April 1972.
More recently, recovery of silver from spent solutions has been made simpler due to introduction of metal salts, most commonly the trisodium salt of trimercapto-S-triazine or TMT, as the metal precipitating agent. Other cationic salts of TMT also can be used as precipitating agents, such as the potassium, ammonium or lithium salt. TMT can be used for primary or secondary recovery of silver; however, many film processors have reported that TMT is very useful and most economical for secondary treatment of spent solutions which have previously had most of the silver removed by metal exchange or electrolysis, for example. In one known method, the spent solution and TMT were mechanically mixed for as long as an hour in a large settling vessel, typically a round-bottomed or cone-bottomed vessel. The resultant mixture was left to settle overnight or for as long as twenty hours. Then, much of the liquid above the settled solids was decanted and the settled solids were passed out of the bottom of the vessel into a bag filter. Some work has been reported in the literature in which a polymeric flocculant has been added to the mixture prior to settling. Methods of the latter type were described by Nathan Spears and Robert Sentell in a paper entitled "Silver Recovery from Photographic Waste Processing Solutions by Using the Trisodium Salt of 2,4,6-Trimercapto-S-Triazine," presented at the Seventh International Symposium on Photofinishing Technology in San Francisco, Calif. 3 to 5 Feb. 1992.
Those skilled in the photographic processing technologies will understand that various other types of components have been removed from spent processing solutions by precipitation, such as Prussian blue (iron ferrocyanide), calcium sulfate, various coupling agents, chromium hydroxide from bleach and systems cleaners, aluminum salts and many others. Some of these precipitated materials tend to form rather gelatinous solids that will quickly clog or blind most filters. Others produce a very large amount of suspended fine particles that tend to remain suspended in the liquid even after rather long settling times.
While such known methods for removing components from waste photoprocessing solutions have proven relatively effective at recovery of precipitated solids, a number of problems have remained. Considerable care has been required when decanting the last portions of the liquid in the zone closest to the settled solids in the bottom of the settling vessel, since the solids tend to stir up and carry out with the liquid, potentially requiring a further filtering operation or return of the liquid and fines from that zone to the vessel for processing with the next batch of spent solutions.
Another problem may exist when solids already settled on the bottom of the settling vessel are disturbed when a valve is opened at the bottom of the vessel to dump the moist solids for further processing. If no liquid remains in the vessel when the valve is opened, difficulty may be encountered with getting the settled solids to flow completely out of the vessel without subsequent scraping or other manual handling. So, enough liquid often has been left in the vessel to permit the solids to be discharged as a slurry for easy conveyance.
Similarly, there may occur times when it is desired to remove solids from the settling vessel while a considerable volume of liquid remains above the settled solids. In such situations, when the solids are disturbed by opening the bottom valve, some solids, particularly fines, are stirred up and resuspended in the liquid layer for a considerable period of time until the entire contents of the vessel are quiescent for a long enough period to allow the fines to settle again. But, if the liquid must be decanted before the fines have resettled, the fines are carried out and a further filtering may be needed to achieve the desired level of purity of the discharged liquid.